Method and device for heat treatment of waste products

ABSTRACT

An apparatus and method are for the pyrolysis of waste. The aparatus has a rotating cell formed of a cylinder in combination with a truncated cone rotating on the same axis. The apparatus also has a hopper for charging the waste at one end of the cylinder, an ash box at the other end of the cylinder, a gas recuperator, and a retaining threshold between the cylinder and the truncated cone. A region of intimate contact of the waste with itself is created, whereby the waste is converted into coke which is used as fuel in the pyrolysis of the waste.

[0001] The subject-matter of the present invention is a process for theheat treatment of waste, in particular, but not exclusively, domesticand industrial waste, and the plant for its implementation, of the typecomprising a rotating combustion cell into which the waste is introducedvia a charging end, whereas the slag is collected at the other end,while the gases are recovered upstream or downstream of the cell.

[0002] Currently, in this type of rotary kiln, the waste is combustedexothermally by introducing oxidizing air, the effect of which is tobring combustion of all the hydrocarbonaceous materials present in thewaste to completion, the stirring of which, during its displacementtowards the ash box, is provided by the rotation of the cylindricaland/or conical combustion chamber.

[0003] At this stage in the combustion, the gases are 99% incineratedand the clinker has a content of uncombusted material of 2 to 10% in theform of carbon.

[0004] The reduction by oxidation of virtually all the hydrocarbonaceousmaterial is reflected by high temperatures of more than 1200° C. forwaste with a mean net calorific value (NCV) of 2000 kcal/kg, whichtemperatures can reach more than 1400° C. with waste with an NCV of 3500kcal/kg and more.

[0005] These high temperatures result in the following phenomena:

[0006] 1—The dust, suspended by the forced aeration of the waste whichis found in the fumes, melts and is deposited on the walls of the kilnand of the boiler.

[0007] 2—The clinker is also molten and agglomerates on the walls.

[0008] To avoid these phenomena, there exists only one solution, tointroduce excess air, which air does not participate in the combustionbut has the role of moderating the combustion temperatures toapproximately 850-900° C.

[0009] However, this excess air exhibits the following disadvantages:

[0010] 1—It requires energy in order to be produced and extracted.

[0011] 2—The volume of fumes generated is greater and requires gas lineswith greater cross sections and volumes.

[0012] 3—The toxic and polluting components which are found in the wasteare virtually completely entrained in the fumes and require alarger-volume and more complex device in order to scavenge them.

[0013] 4—International regulations, which are increasingly restrictive,lay down temperatures for the combustion of fumes which are greater than1150° C. and very low contents of pollutants and dust, which are moreparticularly generated by combustion with excess air.

[0014] 5—The fumes produced at 900° C. only make possible an efficiencyof 60 to 65% for heat recovery, whereas it is desirable to achieve 80 to85%.

[0015] 6—In a combustion chamber operating with excess air, it is verydifficult, if not impossible, to bring under control the energycontribution supplied by waste with an NCV of greater than 3500 kcal/kg.

[0016] To overcome these disadvantages, a novel heat treatment processhas been devised, using pyrolysis, which makes it possible, in theabsence of oxygen, to distil all the waste at a low temperature of theorder of 600 to 700° C., whatever its NCV.

[0017] This novel process is also targeted at producing fuel gases, richin CO, CH₄ or various tars, which are incinerated at 1200° C. underspontaneous combustion in a specific chamber. These gases, which onlyhave a slight load of pollutants because of the low temperatures,require a treatment which is markedly less complex and which is markedlysmaller in scale than the incineration.

[0018] To implement this process, the plant comprises a rotating cellcomposed of a cylinder and of a truncated cone rotating on the sameaxis.

[0019] Pyrolysis is carried out in the cylindrical part of the cell and,as it is energy-intensive, energy is supplied by the coke produced byitself, the combustion of which takes place in the truncated cone,defined as being the generator of heat for the pyrolysis. The thermalreduction, pyrolysis/combustion of the coke, takes placecountercurrentwise, the gases produced moving countercurrentwise to thesolids.

[0020] To allow pyrolysis, it is necessary to have available in the cella region in which the waste in the course of distillation is heated bythe thermal energy originating from the abovementioned generator. At acertain stage in its heating, the waste needs to be brought intoprolonged intimate contact with itself in order to be converted intocoke.

[0021] The fundamental characteristic of the plant for the heattreatment of waste in question lies essentially in the tact that itcomprises this region of intimate contact of the waste with itselfdetermined by a retaining threshold lying between the cylindrical partand the frustoconical part of the rotating cell.

[0022] This is because the waste in the course of coking is forced, incrossing this threshold, to form a volume in which the constituents arebrought mutually into close contact while receiving a small amount ofoxygen. At this instant, the reaction temperature of the waste rises toapproximately 700° C. This retaining of the waste, artificially createdby the threshold, makes it possible to obtain a coke which is used inthe generator cone as fuel to provide the hot gas flow necessary for thepyrolysis.

[0023] In this cone, in a known way, the combustion air is distributedunder the ignited coke by a network of nozzles fed via channels.

[0024] The invention is described below with the help of an example andof references to the appended drawing, in which:

[0025] The single FIGURE is a diagrammatic view of the plant for theheat treatment of waste according to the invention.

[0026] In the drawing, the indicator 1 denotes the rotating cell drivenin rotation by mechanical means represented diagrammatically by thereferences 2.

[0027] Upstream of the cell 1, the arrow 3 denotes the hopper forcharging waste, equipped with a flap 4 and a pushing device 5.

[0028] The chimney for recovering the pyrolysis gases is denoted by theindicator 6.

[0029] An ash box 7 for discharge of the slag or of the coke, symbolizedby the arrow 8, is positioned downstream of the cell 1.

[0030] It is obvious that the fittings and other devices, such as thecharging hopper, the recovery chimney, indeed even the ash box, areknown components which are chosen according to the results to beobtained.

[0031] The rotating cell 1 is composed, according to the invention, of acylindrical part 9, constituting the pyrolyser, in combination with afrustoconical part 10, forming the generator. Between the cylinder 9 andthe truncated cone 10 lies a region 11 connecting the end 12 of thecylinder 9 and the large base 13 of the truncated cone 10. This region11 constitutes a retaining threshold for the waste assuming a highconicity resulting from the difference in diameter between the cylinder9 and the truncated cone 10.

[0032] A network of nozzles fed via distribution channels withcombustion air is provided in the frustoconical part 10. Arrows 14symbolize this air supply.

[0033] It is found that, by virtue of this plant for the treatment ofsolid waste with an NCV ranging from 1500 to 10,000 kcal/kg, thefollowing are obtained:

[0034] the production of a pyrolysis gas with an NCV of 900 to 1100kcal/Sm³, which gas is only slightly polluted and has a high degree ofenhanced value, which is carried out under the best conditions.

[0035] compliance with the strictest antipollution standards withreduced means.

[0036] a substantial reduction in the size and the cost of theinstallations.

1. Process for the heat treatment of waste, in which pyrolysis iscarried out in order to distil, at low temperature (600 to 700° C.) andin the absence of oxygen, all the combustible waste, whatever its netcalorific value (NCV), and in order to produce coke and fuel gases, richin CO, CH₄ and various tars, which can subsequently be incinerated at1200° C. under spontaneous combustion in a specific chamber,characterized in that the hot gas flow necessary for the pyrolysis ofthe waste in the course of distillation is provided by the combustion ofthe coke with a deficiency of air, the gases produced movingcountercurrentwise to the solids.
 2. Plant for the heat treatment ofwaste for the implementation of the process according to claim 1,comprising a rotating cell (1) formed of a cylinder (9) in combinationwith a truncated cone (10) rotating on the same axis and comprising ahopper for charging the waste (3) at one end, an ash box (7) at theother end and a gas recuperator (6), characterized in that a retainingthreshold (11) lies between the cylindrical chamber (9) and thefrustoconical chamber (10), creating a region of intimate contact of thewaste with itself during coking, where it receives a small amount ofoxygen in a substoichiometric amount, in order to convert it into cokewhich is used as fuel in the pyrolysis of the waste.
 3. Plant accordingto claim 2, characterized in that the retaining threshold (11) for thewaste connecting the cylinder (9) and the truncated cone (10) is formedby the difference between the diameter (12) of the cylinder (9) and thediameter (13) of the large base of the truncated cone (10).
 4. Plantaccording to claim 2, characterized in that the truncated cone (10) ofthe cell (1) comprises a network of nozzles fed via channelsdistributing the combustion air (14) in a substoichiometric amount underthe ignited coke in order to provide the heat flow necessary for thepyrolysis.